Comparative study of the failure mechanism of atmospheric and suspension plasma sprayed thermal barrier coatings
Introduction
Ceramic Thermal Barrier Coatings (TBCs) are widely used in hot sections of land-based gas turbines and aero-engines to increase the efficiency and extend the life of metallic components [[1], [2], [3]]. The primary function of the TBC system is to provide a low thermal conductivity barrier, allowing the temperature of metallic components to be kept moderate while increasing the operating gas temperature. Nowadays, YSZ-based TBCs are most widely used and fabricated either by Plasma Spraying (PS) or Electron Beam Physical Vapor Deposition (EB-PVD). Application of each method results in dissimilar and distinctive coatings microstructures. The PS coatings consist of lamellae of rapidly solidified YSZ particles called splats, and have low thermal conductivity due to a myriad of inter-splat voids and microcracks, while EB-PVD coatings typically show columnar microstructure, which exhibits low stiffness and high strain tolerance [[4], [5], [6], [7]].
Recently, a new method which might combine the benefits of both PS and EB-PVD has emerged. In Suspension Plasma Spraying (SPS) [[8], [9], [10]] particles in the size range of sub-micrometer to nanometer are suspended in water or organic solvent are used as a feedstock to realize fine microstructure with sub-micrometer splats and nano-porosity. These attributes of microstructure may greatly reduce the coating's thermal conductivity [11,12]. Previous studies have also demonstrated that various coating structures can be realized by the SPS process, including vertically cracked and columnar microstructure [13,14], which may result in significant improvement of compliance, durability, and lifespan of YSZ TBC. Despite these promising features and close attention paid to various aspects of SPS process, only a limited number of publications have focused on performance-related properties for TBC, such as thermal shock resistance or thermal stability [12,[15], [16], [17]]. There are even no unequivocal results showing the advantage of SPS coatings in thermal cycle lifetime over APS coatings. Results of several works dealing with thermal shock resistance of SPS coatings are summarized in Table 1.
Results presented in Table 1 shows very big discrepancy between results for SPS coatings even if they were reported in the same paper. Such a considerable variation may indicate that the performance of SPS coatings are highly sensitive to their microstructure [21] and mechanical properties, which in turn can significantly vary depended on deposition parameters such as substrate [12], size of ceramic powder [18,22], and concentration of solid phase in suspension [18]. Abundance of factors affecting SPS coating's performance makes difficult to determine differences between failure modes of SPS and APS or EB-PVD. However, some researchers made an effort to explain observed differences Zhai et al. [20], who have reported a superior lifetime of SPS coatings due to a columnar or segmented microstructure, which reduces thermal stress in a manner similar to the EB-PVD coatings. Also, results presented by Zhao et al. [19] showed a longer lifetime of SPS coatings and underlined that failure modes of the SPS and APS could be different. The SPS coatings failed by partial spallation of segments/columns, while the APS coatings failed by the bridging of interface delamination close to the bond coat/top coat interface and the intrinsic segmentation cracks. Other investigators have reported the opposite trend of APS coatings outperforming SPS ones. Such results were presented by Seshadri et al. [21] and Ganvir et al. [22], the latter of which suggested that the reason could be the high magnitude of deposition stresses, randomly oriented columnar features or presence of interpass porosity bands. To the best of the authors' knowledge, there is a very limited number of published study aimed at understanding of SPS coating's failure in a comprehensive manner.
Because the failure mechanism of SPS TBC has not been clarified yet, the current study aims to closely investigate this issue. Several SPS TBCs were sprayed with various spraying conditions and then tested in thermal cycle test. Microstructure, mechanical properties and residual stresses of the coatings were investigated in order to understand the thermal cycle performance of the SPS coatings.
Section snippets
Feedstock materials and spraying conditions
Two types of suspension of YSZ (8 wt% Yttria-Stabilized Zirconia) were used in this study. One of which was ethanol-based suspension with the powder loading of 25 wt%. For the suspension preparation commercially available fused and crushed powder (YSZ, Imerys Fused Minerals, Laufenburg, Germany) with a particle size < 2 μm (Fig. 1) and reagent grade ethanol (99.5%) as the solvent was used. The suspension was prepared by mixing the YSZ powder with ethanol, followed by ball milling for 24 h by
Microstructure
Cross-sectional microstructures of the as-sprayed coatings are presented in Fig. 3 with high-magnification images in the insets. For the APS coatings typical lamellar microstructure without vertical cracks is shown in Fig. 3a. At higher magnification pores and intersplat voids are visible. Microstructures of C1-50E and C2-70W coatings are presented in Fig. 3b and f, respectively, both of which exhibit relatively dense microstructure with deep vertical cracks (labeled “A”). Fig. 3b inset reveals
Discussion
There exists a large number of reports describing properties affecting the lifespan of TBCs under thermal cyclic or thermo-mechanical fatigue conditions. Many authors have emphasized the role of TGO growth and thermal expansion mismatch between the bond coat, TGO, and YSZ top coat, leading to high magnitude of residual stress in the TGO layer (1 GPa and more) and subsequent crack nucleation and growth [50]. Other researchers have found the cause of failure in sintering driven stiffening [64],
Conclusions
In this study, several SPS coatings were sprayed with various spraying conditions (spraying distance, plasma, output power, and by using two types of suspensions). Most of the SPS coatings failed after a smaller numbers of cycles than the benchmark APS coating. The majority of SPS coatings show short lifespan in spite of apparently columnar or segmented microstructure. Only one SPS coating – C1-70E – performed comparatively with the APS coating. Link between thermal cycle performance and
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